First things first. Go over to Michael Welland's blog - Through the Sandglass and congratulate him. His superb book Sand:The Never-Ending Story has been awarded the John Burroughs medal for excellence in natural history writing.
Sometime back Michael wrote a couple of posts on Tafoni, the beautiful enigmatic natural rock weathering sculptures found commonly in sandstones but also in other types of lithologies. The images that stuck with me from his posts were of desert landscapes and sandstone sculptures....so I got a pleasant surprise when I came across Tafoni in basalt on my recent field trip to the west coast.
Tafoni are ellipsoidal..bowl shaped natural rock cavities...go here to learn more.
They commonly occur in locations where salt weathering is intense. Water percolates through natural cracks in rocks and precipitate salts. These crystallizing salts exert pressure on the surrounding rock and little by little the host rock gives way to form holes and pits. Besides salt weathering the natural variation in hardness in rocks masses may also promote this differential weathering.
The location was just south of the town of Alibag ( Alibag incorrectly shown...it is directly west of the annotation... on the coast), near Korlai fishing village.
There are wave cut basalt benches exposed at low tide which are intruded by dikes. These basalt flows are studded with secondary silica and zeolites mineralization and ...Tafoni.
Here are some examples:
A close up of the beautiful honeycomb structure
Pits and closely spaced joints..
Associated silica and zeolite geodes
Pits...large and small..
Cluster of Tafoni on vertical and inclined surfaces..separated by regions which lack Tafoni..
Besides being located in the intertidal zone where constant wetting and drying and salt weathering is strong..there may be a couple of other reasons why Tafoni is localized to these flows.
First, these flows are not one homogenous rock mass. They are compound flows...i.e. made up of smaller flow units...small and big blobs of lava that overlap, interfinger and coalesce to form a larger compound flow representing one macro eruptive episode. That means there are flow unit scale variations in cooling rates and vesicular content that makes individual flow units unequally suseptible to weathering.
And second, is the presence of dikes. Not directly..but these dike swarms are concentrated in regions where the crust has undergone extension...leaving the rocks riddled with closely spaces fractures.
That makes them vulnerable to dissolution through circulating water and chemical weathering.